1. Field of Invention
The invention relates generally to methods and systems for nondestructive evaluation and/or structural health monitoring and, more particularly, to a method and system for multi-path active defect detection, localization, and characterization using ultrasonic guided waves.
2. Discussion of Related Art
There is a well-recognized need for rapid and reliable methods to inspect large-area plate-like structures such as, for example, metallic and composite aerospace components, marine vessel hulls, and civil, nuclear, and petrochemical infrastructure. Ultrasonic guided waves have been identified and utilized for this purpose because they travel for long distances in the plane of the structure and are sensitive to both surface and sub-surface features.
Several known methods have been developed to perform nondestructive evaluation (NDE) and/or structural health monitoring (SHM) utilizing ultrasonic guided waves. Known elliptical and hyperbolic methods, for example, use time-of-arrival or time-difference-of-arrival information, respectively, to identify elastic scattering from defects or damage. For example, C. H. Wang, J. T. Rose, and F.-K. Chang, “A synthetic time-reversal imaging method for structural health monitoring,” Smart Materials and Structures, 13 (2), pp. 415-423 (2004) and J. E. Michaels, A. J. Croxford, and P. D. Wilcox, “Imaging algorithms for locating damage via in situ ultrasonic sensors,” in IEEE Sensors Applications Symposium, pp. 63-67 (2008), both of which are hereby incorporated by reference. Alternatively, as disclosed, for example, in P. Malinowski, T. Wandowski, I. Trendafilova, and W. Ostachowicz, “A phased array-based method for damage detection and localization in thin plates,” Structural Health Monitoring, 8 (1), pp. 5-15 (2009), hereby incorporated by reference, phased arrays have been shown to identify scatterers using beamforming techniques.
A common characteristic among most known methods developed to date is that they inherently assume a homogeneous, isotropic medium with a clear, direct path between the transducers and the interrogation structure. To balance the needs of weight, function, and cost, however, practical large-area structures are increasingly being constructed of inhomogeneous or anisotropic materials and often contain a large number of structural features such as, for example, stiffeners, ribs, cut-outs, fasteners and the like. Consequently, the underlying assumptions of state-of-the-art ultrasonic guided wave defect detection and localization algorithms are no longer practical assumptions.
In complex structures, guided waves often travel along indirect paths and via multiple modes between transmitter and receiver. If damage is introduced in such a structure, signal changes caused by the damage may be readily observed, but with existing methods it can be impossible to relate the changes to a specific location, or determine the type and severity of the damage. A known approach involves high sensor density in an attempt to ensure that there are sufficient direct paths between transducers and every location of interest on the structure that pass through an essentially homogeneous material. While such an approach may be technically viable, it is usually not economically viable because of the added cost and weight. A further complication is the impracticality of directly modeling ultrasonic waves propagating in a truly complex structure. Even if modeling were practical, unavoidable deviations between the as-built and modeled structures prohibit direct comparisons of modeled data with actual measurements.